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Flying boats

The 34th America’s Cup is over, and after a miracle occurred, Oracle Team USA won!

I give it to the Oracle team, they recovered from the shock of Team New Zealand’s (TNZ) initial skill and speed, and climbed the steep learning curve like a monkey! They were clearly the faster boat by the last week of the competition. It was TNZ’s to lose, and Oracle’s to win, and they did it.

While I was watching the event, I got quite interested in how the boats sail so fast, especially how they “foil” (short for hydrofoil.) But before I learned about that, I had to figure out how boats sail against the wind, and faster than the wind.

How do boats (any and all) sail upwind? I saw a video during the AC34 races where a guy explained it like pinching a wedge shaped object between your fingers. The sail/wing produces a force trying to push the boat over (sort of sideways) but the keel (and did you see how small the keels (rudders and daggerboards on the cats) were) which is slicing through the water having a substantially greater coefficient of friction than air, is exerting an equal force on the boat to make it stand upright. Two squeezing forces. Put a marble between your finger and thumb and squeeze together–the marble launches away. So does the hull of the boat. It accelerates forward. Consider now the forces that must be exerted on the boat to be able to make it move forward at 50mph!

As for foiling, the dagger board is J shaped and so there’s a horizontal surface under the water that produces lift as the boat moves forward. The crew can control the angle of the board to lift the boat (going straight the force is about 7 tons on a piece of carbon fiber the size of a surfboard) out of the water. Less drag from the hull…go faster, produce more lift…oops but not too much! Spithill, Barker and the crews made it look easy, but they must be on the edge every second.